This invention relates to optical storage systems such as optical disks, cards, etc. which are of ROM (Read-Only-Memory), WORM (Write-One-Read-Many) or Rewritable types. In particular, the present invention relates to such system which utilize fluorescent materials.
The following references are considered to be pertinent for the purpose of understanding the background of the present invention:
1. WO 98/50914;
2. WO 99/24527;
3. xe2x80x9cMicro-Optics, Elements, Systems and Applicationsxe2x80x9d, Ed. H. P. Herzig, Taylor and Francis, p. 153 (1997);
4. WO 98/28739;
5. WO 98/28740;
6. WO 98/31018;
7. U.S. Pat. No. 5,847,141;
8. Shvartsman F. P. (1993). Replication of Diffractive Optics, Lee S. H. (ed.) Diffractive and Miniaturized Optics, Vol. CR49, pp. 117-137. Bellingham: SPIE.
9. U.S. Pat. No. 4,090,031.
The above references will be acknowledged in the text below by indicating their number in brackets.
The attractive unique features of optical storage devices are their high capacity, removability, mass replicability and long memory persistence for archival applications. They are most commonly used for software distribution, backup memory for personal computers and workstations, external memory for some mainframes and as large-capacity off-line memory. However, storage capacity and signal-to-noise ratio of the existing optical storage devices are not sufficient for the newly developed computers and new generation video systems since such devices typically comprise only one information-carrying layer. Therefore, materials for more advanced optical systems with increased information-recording density, high level of signal-to-noise, high resolution and low cost are worked up intensively.
The storage media of most optical storage devices are in the form of a rotating disk. In general, the disks are preformatted using pits (grooves) and gaps to enable positioning an optical pickup and recording head to access information on the disk. A focused laser beam emanating from the optical head records information on the media as a change in the material characteristics.
Production of multi-layered memory carrier with fluorescent reading shows promise for increasing capacity of optical-storage medium. A method for constructing a multi-layered optical fluorescent disc is disclosed in [1], in accordance with which replicas of single-layer discs are produced by the Injection Molding method, where pits are filled with a fluorescent, dilute composition. The single layer discs are stuck one to another, so that active layers consisting of fluorescent pits with a depth of 0.5 xcexcm are interleaved with non active separation layers of 20-50 xcexcm depth and which are transparent to wavelengths of exciting laser and fluorescent light. Fluorescent compositions for use in the above-mentioned multi-layer disc have been suggested [2].
The basic requirements for such fluorescent compositions are: suitable penetration into pits without dying the spaces between them, inactivity towards substrate materials such as polycarbonate (PC) or polymethylmethacrylate (PMMA) and the quantum output of the fluorescent material used in such formulations must be as high as possible and must not decrease during long-term storage and operation.
These requirements significantly restrict the choice of the components of a fluorescent composition, namely, film-forming polymers, plasticizers and solvents, to such components which are not aggressive towards substrates made of PC or PMMA. Another obstacle hampering the mass production of multi-layer fluorescent discs, by the injection molding method, is the technological complexity of manufacturing single-layer discs with a thickness of 20-50 xcexcm and containing pits with a depth of 0.5 xcexcm.
There are known in the art replication techniques such as hot embossing or thermo- or UV-casting/embossing [3]. Such replication techniques are already used for the commercial production of submicron grating structures (hot embossed diffractive foil and security holograms). However, replication processes based on hot embossing into thermoplastic resins, or on casting/embossing with thermosetting resins or UV-curable polymers have not been applied for production of multi-layered optical information carriers with fluorescent reading of ROM, WORM and Rewritable types.
The possibility of obtaining multi-layer optical data carriers by mechanical printing, by using photoluminescent materials was described in [9]. However, there is no information in this patent regarding a structure of multi-layer carrier materials, as well as a specific production method, that would allow the implementation in practice of the method for manufacturing a multi-layer carrier of fluorescent memory with the required parameters.
The production of the above-mentioned products (media) by techniques of hot embossing or thermo- or UV-casting/embossing depends on several parameters, among them are the following:
1. Development of fluorescent compositions suitable to produce ROM, WORM, and Rewritable type information carriers and methods of deposition of such fluorescent compositions on flexible substrates.
2. Formation of surface relief in a form of a pattern or a plurality of discrete portions of an active fluorescent material (pit, spiral, track, ect.) by the methods of hot embossing or thermo- or UV-casting/embossing.
3. Filling the recesses of a patterned structure with polymeric materials in order to eliminate diffraction and to obtain a transparent layer containing fluorescent discrete structures (pits, tracks, etc.).
4. Sticking together single-layer films carrying fluorescent optical information in order to obtain a multi-layer optical carrier.
The above-mentioned parameters are dealt with in the present invention.
By taking the above-mentioned into consideration, a major object of the present invention is to provide a method for manufacturing multi-layer fluorescent optical information carrier of ROM, WORM and Rewritable memory types in the form of CDs, cards or similar products, such carriers providing high density of optical memory, high resolution, high fluorescence signal and high signal-to-noise ratio.
It is a further object of the present invention to provide such a method which is suitable for manufacturing fluorescent optical information carriers, having high stability and quality of information playback, reduces the cost of the media and of the device of reading of fluorescent signals.
There is thus provided according to one aspect of the present invention a method of manufacturing a multi-layer optical information carrier with fluorescence reading/recording, the method comprising the steps of:
(i) fabricating a structure formed of a substrate carrying a fluorescent layer on at least one surfaces thereof, the substrate being transparent with respect to incident radiation used for the fluorescence reading/recording;
(ii) subjecting said structure to a replication technique such as to produce a fluorescent patterned structure with a surface relief in the form of an array of discrete fluorescent regions;
(iii) repeating steps (i) and (ii), so as to obtain a plurality of fluorescent patterned structures,
(iv) joining together said plurality of separate fluorescent patterned structures so as to form a multi-layer optical information carrier.
The multi-layer optical information carriers produced by the above method constitute another aspect of the invention.
Thus, according to a preferred embodiment of the invention, the method of the present invention contemplates the use of fluorescent compositions which comprise a thermoplastic polymer capable of passing into viscous-fluidic state at increased temperature and an organic dye or mixtures of dyes, in order to perform the formation of a surface relief by the method of hot stamper or roller embossing. The organic dye is selected according to the specific type of the information carrier to be manufactured, namely a dye suitable for a fluorescent ROM material [2], or dyes and appropriate additives according to [2, 4, 5] suitable for a fluorescent WORM material, or photochromic substance according to [6, 7] suitable for a fluorescent Rewritable material.
In accordance with another embodiment, the method of the invention contemplates the use of a fluorescent composition, consisting of a thermo-setting or UV-curable material able to be hardened under conditions of increased temperature, or exposure to UV irradiation, and an organic dye, in order to form a patterned structure by the method of stamper or roller embossing or thermo- or UV-casting/embossing. The organic dye may be selected, as explained above according to the specific type of the information carrier to be manufactured.
In the method of the invention, the recesses of a patterned structure produced by hot embossing or thermo- or UV-casting/embossing are filled with a transparent polymer substance in order to eliminate diffraction and to obtain a transparent layer containing fluorescent discrete structures (pits, spiral tracks, etc.).
Single-layer materials are joined together so that the active layers containing fluorescent structures are interleaved, with separation layers formed by the substrate of each single layer. Single layer materials can also be joined together without preliminary filling of the surface recesses. In this case, the adhesive used when joining together single-layer materials so as to form a multi-layer structure, fills the recesses during the adhering process.